As a braking system for railway vehicles, disc brakes which have excellent braking performance become more frequently used as the speed of vehicle and the size thereof increase. A disc brake is configured such that a brake lining is pressed against a sliding surface of a brake disc attached to a wheel. As a result, braking force is generated in a rotating wheel, thereby controlling the speed of vehicle.
The type of disc brake includes a center-fastening type (sliding-surface fastening type) brake disc in which brake discs are fastened to a wheel in a region within a sliding surface, and an inner-circumference fastening type brake disc in which brake discs are fastened to a wheel in a region located closer to the inner circumference than the sliding surface is. The inner-circumference fastening type brake disc requires a portion to be used for fastening, aside from a portion including a sliding surface. On the other hand, the center-fastening type brake disc does not need to be provided with such a portion to be used for fastening, and therefore is advantageous in weight reduction.
FIGS. 1A and 1B are diagrams to show an overall structure of a railway wheel with brake disc constituting a disc brake for a railway vehicle, in which FIG. 1A shows a plan view of a quarter circle portion, and FIG. 1B shows a sectional view along the radial direction of a half circle portion, respectively. FIGS. 2A to 2C are diagrams to topically show the structure of a conventional railway wheel with BD, in which FIG. 2A shows a perspective view of a back face of a brake disc seen from the inner peripheral surface side, FIG. 2B shows a plan view of a brake disc seen from a back face side, and FIG. 2C shows a sectional view along the radial direction, respectively. All of the brake discs shown in FIGS. 1A, 1B, and 2A to 2C are of center-fastening type.
As shown in FIGS. 1A, 1B, and 2A to 2C, a brake disc 1 includes an annular circular plate portion 2 whose front face 2a provides the sliding surface. In the back face 2b of the circular plate portion 2, a plurality of fin portions 3 are projected in a radial manner. In some of the plurality of fin portions 3, a bolt hole 4 passing through to the circular plate portion 2 is formed at an approximately central position in the radial direction.
A wheel 10 includes a boss portion 11 into which an axle shaft is to be press fit, a rim portion 12 including a tread which is to be in contact with a rail, and a plate portion 13 for combining them together. The brake discs 1 are disposed in a set of two so as to interpose the plate portion 13 of the wheel 10 therebetween with the front face 2a of each brake disc facing outwardly. A bolt 5 is inserted into each bolt hole 4, and a nut 6 is screwed onto each bolt 5 and fastened thereto. As a result, the brake disc 1 is fastened to the wheel 10 in such a way that a front end surface of the fin portion 3 is in pressure contact with a side face 13a of the plate portion 13 of the wheel 10 over the entire range of the radial direction.
The brake disc 1 is fastened to the plate portion 13 in a region within the sliding surface. In the center-fastening type brake disc, regarding the radial direction of the brake disc 1, it is preferable that the brake disc 1 is fastened to the wheel 10 in the vicinity of a central portion between the inner circumference and the outer circumference of the brake disc 1, for example, a portion between a position to internally divide the inner circumference and the outer circumference into a ratio of 1:3, and a position to internally divide the inner circumference and the outer circumference into a ratio of 3:1.
Substantially the entire surface of the front face of the brake disc 1 serves as a sliding surface, and as shown in FIG. 1B, a large gap (for example, a gap of 70 to 120 mm) is formed over the entire circumference between the boss portion 11 and the circular plate portion 2. That is, the brake disc 1 does not extend to the vicinity of the boss portion 11, thus realizing weight reduction of the brake disc 1. A conventional railway wheel with BD having such a configuration is disclosed in, for example, Patent Literature 1.
While a railway vehicle is travelling, the brake disc 1 rotates integrally with the wheel 10 at a high speed. Accordingly, air around the brake disc 1 flows from the inner circumference side (a gap between the boss portion 11 and the circular plate portion 2) into a space formed between the brake disc 1 and the wheel 10, specifically, a space surrounded by the circular plate portion 2 and the fin portion 3 of the brake disc 1, and the plate portion 13 of the wheel 10, and flows out from the outer circumference side (see solid arrows in FIGS. 2A to 2C). That is, while the railway vehicle is travelling, a gas flow of air occurs in a space between the brake disc 1 and the wheel 10. Such a gas flow becomes significant when the vehicle travels at a high speed of more than 300 km/h, like a high-speed railway vehicle such as the Shinkansen (R), thereby inducing noise referred to as aerodynamic sound. For that reason, reduction of aerodynamic sound is required from consideration of the environment.
In an inner-circumference fastening type brake disc, the gap between the boss portion 11 and the circular plate portion 2 is very small compared with a center-fastening type brake disc. For that reason, while the vehicle is travelling, the amount of air that flows into the space surrounded by the circular plate portion 2 and the fin portion 3 of the brake disc 1, and the plate portion 13 of the wheel 10 is small, and generally aerodynamic sound at a level that causes a problem will not be generated. Therefore, it can be said that the generation of aerodynamic sound is a problem peculiar to the center-fastening type brake disc.
Prior arts that address the need to reduce aerodynamic sound associated with a center-fastening type brake disc include the followings.
For example, Patent Literature 2 discloses a railway wheel with BD, in which a brake disc is additionally provided with a rib between adjacent fin portions along the circumferential direction so that the gas flow is suppressed by the rib. According to the railway wheel with BD disclosed in the above described literature, it is possible to reduce the aerodynamic sound to a desired level.
However, in the technique disclosed in Patent Literature 2, cooling performance for the brake disc during braking deteriorates as the gas flow is suppressed by the rib. For this reason, increases in the deformation caused by thermal expansion of the brake disc, and in the stress load inflicted upon the fastening bolt thereby, coupled with increase in the stiffness of the brake disc itself due to the addition of the ribs may cause a risk that durability of the brake disc and the bolt deteriorates.
A prior art to solve this problem is disclosed in Patent Literature 3.
FIGS. 3A and 3B are diagrams to topically show the structure of a conventional railway wheel with BD disclosed in Patent Literature 3, in which FIG. 3A shows a perspective view of a back face of a brake disc seen from its inner peripheral surface side, and FIG. 3B shows a sectional view along the radial direction. As shown in these figures, in the railway wheel with BD disclosed in Patent Literature 3, the brake disc 1 is added with ribs 7 in the circumferential direction each between adjacent fin portions 3, and further a slit 7a is formed along the radial direction in a central portion in the circumferential direction of each rib 7.
According to this railway wheel with BD, a gas flow is ensured by the slit 7a. Since this allows to maintain the cooling performance for the brake disc 1 during braking, and to lighten the increase in stiffness due to addition of the rib, deformation accompanying thermal expansion of the brake disc 1 and stress load inflicted on the fastening bolt are mitigated, thereby suppressing the deterioration in the durability of the brake disc 1 and the bolt.